(1) Field of the Invention
This invention relates to a method of removing a deck from an offshore structure and to a vessel suitable for use in such a method.
There are many structures, for example in the North Sea, that have been built and installed on the seabed for purposes connected with the offshore oil and gas industries. Such structures commonly comprise a supporting framework, usually referred to as a jacket, which stands on the seabed and extends up to a height above sea level, and a superstructure, often referred to as a deck, supported above sea level on the jacket. A jacket typically comprises a plurality of legs extending upwardly from the seabed to the top of the jacket and diagonal and cross bracing that together hold the legs against relative lateral movement; the vertical load carried by the jacket is borne principally by the legs. The nature of the deck is dependent upon the purpose of the structure. For example, it would commonly comprise principally a drilling rig but might consist exclusively of accommodation for workers on an adjacent rig. During installation, the jacket is commonly located in position on the seabed first and the deck thereafter placed on top of the jacket. The deck may be built as a single unit onshore, taken out to sea and placed on top of the jacket, or it may be built as a number of separate modules that are taken separately to the jacket and assembled only as they are placed on the jacket. Modules can also be added to a deck that has previously been placed on a jacket at a later stage to enhance or alter the capabilities of the deck.
It will be appreciated that the form of superstructure and the form of the supporting structure vary considerably from one structure to another and the terms xe2x80x9cdeckxe2x80x9d and xe2x80x9cjacketxe2x80x9d as used herein need to be understood as correspondingly broad.
(2) Description of Related Art
As environmental considerations assume greater importance, so the need increases for satisfactory methods of removing a deck from a jacket of an offshore structure after the useful life of the structure is over. One way that may be adopted is to use a vessel with a large crane to lift the deck from the jacket and place it on a barge. Many other options have, however, also been proposed and in some cases also used in practice; in some of these options a floating vessel, which in plan view is generally U shape, is moved up to the structure with the opposite limbs of the xe2x80x9cUxe2x80x9d on opposite sides of the structure and some system, which may be a ballasting or a jacking system, used to lift the deck clear of the jacket.
In practice, however, it has proved difficult to provide a method of removing a deck from an offshore structure that (1) is able to remove a relatively large and heavy deck, (2) is able to bring the deck inshore all the way to a yard and (3) does not require a very great investment in equipment.
It is an object of the invention to provide a method of removing a deck from an offshore structure that overcomes at least partly some or all of the difficulties referred to.
According to the invention there is provided a method of removing a deck from an offshore structure including a deck supported on a jacket, the method including the following steps:
a) positioning a floating vessel around the jacket with respective parts of the vessel on opposite sides of the jacket and trusses extending between the opposite parts of the vessel,
b) engaging parts of the trusses with legs of the jacket,
c) securing the trusses to the jacket,
d) relieving the load carried by portions of the legs of the jacket,
e) cutting through the portions of the,legs of the jacket to divide the jacket into a lower part and an upper part carrying the deck, the trusses being secured to the upper part of the jacket,
f) transferring the weight of the upper part of the jacket and of the deck via the trusses to the floating vessel, and
g) removing the floating vessel, with the trusses, the upper part of the jacket and the deck supported thereon, from the vicinity of the lower part of the jacket.
In the method just defined the steps are set out in one particular order which is the preferred order, but it should be understood that it is within the scope of the invention to make some alterations to the order. For example, it is within the scope of the invention for the step of relieving the load carried by portions of the legs of the jacket to be carried out before the trusses are secured to the jacket.
By cutting through the jacket below its top and then removing the deck by supporting the uppermost part of the jacket, rather than trying to remove the deck from the top of the jacket, a more reliable support of the deck is ensured: by continuing to support the deck through the jacket the nature of the support for the deck itself remains unchanged and therefore there can be confidence of adequate support for the deck; if the deck were removed from the jacket, however, the nature of its support would almost inevitably change and therefore there could be less confidence that its support would be satisfactory, especially in the case of a deck of modular construction and/or a deck to which structural alterations had been made subsequent to installation. Where reference is made herein to xe2x80x9ccuttingxe2x80x9d a leg it should be understood that the term should not be regarded as restricted to any particular method of creating a separation of upper and lower parts of a leg; methods that may be employed and are to be regarded as cutting include, for example, a shearing action, application of heat and explosive methods.
Usually it will be advantageous for the trusses to be secured to the jacket above sea level. Also it will usually be advantageous for the legs to be cut above sea level.
The floating vessel preferably includes two barges, which may or may not be identical, for positioning on opposite sides of the jacket. The barges preferably are able to be separated and be used for another purpose as individual barges. Constructing the vessel in this way enables the cost of the vessel to be reduced.
Preferably the barges are connected together side-by-side with a space therebetween, by front and rear trusses, which preferably are detachably connected to the barges.
One of the trusses may be retractable, preferably by being separated into two parts, to leave an open-ended space between the barges. In such a case, step (a) of positioning the floating vessel preferably includes the sequential steps of retracting the retractable truss, positioning the vessel around the jacket with the barges on opposite sides of the jacket and the jacket positioned within the open-ended space, and returning the retracted truss to a position in which it extends across the gap between the barges on the opposite side of the jacket from the other truss.
As an alternative to the procedure described immediately above, step (a) of positioning the floating vessel around the jacket may include the following steps:
i) positioning the vessel adjacent to the jacket with one of the trusses immediately adjacent to the jacket,
ii) releasing the truss that is immediately adjacent to the jacket from the vessel, and
iii) moving the vessel away from the jacket and then to an opposite side of the jacket and repositioning the vessel around the jacket with respective parts of the vessel on first and second opposite sides of the jacket and the trusses extending between the opposite parts of the vessel on third and fourth opposite sides of the jacket.
The truss that is immediately adjacent to the jacket is preferably mounted on a buoyancy unit which supports at least most of the weight of the truss when the truss is released from the vessel. That avoids the need to have the offshore structure supporting the weight of the truss at this stage.
Preferably the step of releasing the truss that is immediately adjacent to the jacket from the vessel includes the step of adding ballast to the vessel to lower it.
Preferably the vessel is moved to the opposite side of the jacket and repositioned around the jacket in its lowered position and then raised to bring it back into a position in which it supports the truss that was previously released from the vessel.
In the common case where the jacket is of generally rectangular section at sea level, it is preferred that the trusses are positioned alongside the longer sides of the jacket. By enclosing the jacket within the vessel the vessel is assured of remaining in position and is able to be positioned immediately adjacent to all parts of the jacket. Placing the trusses alongside the longer sides of the jacket facilitates the engagement of the trusses with legs of the jacket. It is generally preferred that the trusses engage all the legs of the jacket although in some cases that may not be desirable.
Step (b) of engaging parts of the trusses with legs of the jacket preferably includes moving movable parts of the trusses into engagement with the legs. Preferably each leg is engaged by a part of one of the trusses at two locations vertically spaced from one another. Preferably at least one collar is fixed to each leg as a preliminary step in the method and vertical loads are transferred between the legs and the trusses by the collars. The provision of such pre-installed collars facilitates the effective transfer of the large loads involved, between the legs and the trusses.
Preferably the parts of the trusses that engage the legs include grippers that are able to transfer horizontal loads between the legs and the trusses.
Preferably the method further includes the step of detaching the trusses from the vessel after the trusses are secured to the jacket. Such a step may seem surprising but represents a useful step in the procedure because it enables the time for which there is a fixed connection between the vessel, that is floating on the sea, and the offshore structure, that is stationary, to be kept to a minimum, thereby making it easier to prevent undesirable forces or movements being generated by sea movements. The step of detaching the trusses preferably includes the step of retracting jacks positioned between the vessel and the trusses; it may also or instead include the step of ballasting the vessel.
Step (d) of relieving the load carried by portions of the legs of the jacket preferably serves to reduce the vertical load carried by the portions of the legs to substantially no vertical load. Step (d) preferably involves the steps of placing jacking systems around portions of the legs of the jacket, and actuating the systems such that vertical loads previously carried through the portions of the legs are carried through the jacking systems.
Step (e) of cutting through the portions of the legs of the jacket may include the step of cutting through diagonal bracing of the jacket. As will be appreciated, it is necessary prior to step (g) to have a complete separation of the upper and lower parts of the jacket. It may also be necessary to remove or sever risers, caissons and xe2x80x98Jxe2x80x99 tubes.
Step (f) of transferring the weight of the upper part of the jacket and of the deck via the trusses to the floating vessel may include, in the case where trusses have been detached from the vessel, the step of reattaching the trusses. The transfer of weight may include the step of extending jacks positioned between the vessel and the trusses, and/or the step of deballasting the vessel and/or the step of actuating jacking systems placed around the legs of the jacket. Generally it will be desirable for at least part of step (f) to be carried out relatively quickly as the vessel and the structure are liable to be most exposed to undesirable effects, for example ones caused by sea movements, during, immediately before or immediately after the transfer of weight.
The method may further include the step of taking the vessel to shore and transporting the trusses, with the upper part of the jacket and the deck supported thereon, onto the shore. Such a procedure enables the step of transferring the upper part of the jacket and the deck onto the dry land to be simplified.
The method described above is the most preferred form of the invention. Some of the features described above as being preferred or advantageous rather than essential are in themselves capable of providing considerable advantages in methods which may not include all the features (a) to (g) of the method of the invention described above (referred to hereinafter as the method according to a first aspect of the invention).
Thus according to a second aspect of the invention, there is provided a method of removing a deck from an offshore structure including a deck and a jacket, the method including the following steps:
placing jacking systems around portions of the legs of the jacket,
actuating the jacking systems to relieve the load carried by the portions of the legs of the jacket,
cutting through the portions of the legs of the jacket to divide the jacket into a lower part and an upper part carrying the deck, and
removing the deck from the jacket.
According to a third aspect of the invention, there is provided a method of removing a deck from an offshore structure, the method including the following steps:
providing a floating vessel comprising two barges, connected together side-by-side with a space therebetween, by front and rear trusses,
positioning the vessel adjacent to the structure with one of the trusses immediately adjacent to the structure,
releasing the truss that is immediately adjacent to the structure from the vessel,
moving the vessel away from the structure and then to an opposite side of the structure and repositioning the vessel around the structure with the barges on first and second opposite sides of the structure and the trusses extending between the barges on third and fourth opposite sides of the structure,
transferring the load of the deck via the trusses to the barges, and removing the barges, with the trusses and the deck supported thereon.
According to a fourth aspect of the invention, there is provided a method of removing a deck from an offshore structure, the method including the following steps:
providing a floating vessel comprising two barges connected together side-by-side with a space therebetween, by front and rear trusses,
retracting one of the trusses to leave an open-ended space between the barges,
positioning the vessel around the structure with the barges on opposite sides of the structure and the structure positioned within the open-ended space,
returning the retracted truss to a position in which it extends across the gap between the barges on the opposite side of the structure from the other truss,
transferring the load of the deck via the trusses to the barges, and
removing the barges, with the trusses and the deck supported thereon.
According to a fifth aspect of the invention, there is provided a method of removing a deck from an offshore structure including a deck supported on a jacket, the method including the following steps:
positioning a floating vessel around the jacket with respective parts of the vessel on opposite sides of the jacket and one or more trusses extending between the opposite parts of the vessel,
securing the trusses to the jacket,
detaching the trusses from the vessel,
cutting through the jacket to divide the jacket into a lower part and an upper part carrying the deck, the trusses being secured to the upper part of the jacket,
reattaching the trusses to the vessel and transferring the weight of the upper part of the jacket and of the deck via the trusses to the floating vessel, and
removing the floating vessel, with the trusses, the upper part of the jacket and the deck supported thereon from the vicinity of the lower part of the jacket.
According to a sixth aspect of the invention there is provided a method of removing a deck from a jacket of an offshore structure, the method including the following steps:
bringing a floating vessel to the structure;
engaging legs of the jacket with parts of the vessel;
separating upper portions of the legs of the jacket from lower portions;
transferring an upper part of the jacket that includes the upper portion of the legs, with the deck attached thereto, onto the vessel; and
removing the upper part of the jacket and the deck from the vicinity of the lower part of the jacket.
It should be understood that the method of any of the second, third, fourth, fifth or sixth aspects of the invention may further include any of the advantageous or preferred features referred to above in connection with the first aspect of the invention.
The invention still further provides a vessel suitable for carrying out any of the methods described above.
One example of a suitable vessel comprises two barges connected together side-by-side with a space therebetween, by front and rear trusses, the trusses being detachable from the barges.